Method of dephosphorizing and refining thomas and bessemer steels



METHOD OF DEPHOSPHORIZING AND REFINING THOMAS AND BESSEMER STEELS Arthur Thill, Metz, France No Drawing. Application April 6, 1951, Serial N0. 219,769

' 11 Claims. (CI. 75-55) Iron and steel plants which operate deposits of oolitic iron ore or similar products are presently confronted with the problem of improving the quality of Thomas steel.

Constant efforts have been made in this direction during the last ten or fifteen years in all European countries with more or less success, their purpose being threefold:

(l) The reduction of the degree of waste;

(2) The increase in the productive capacity of the plant; and

(3) The improvement of all the qualities of the metal.

The main causes of the relative depreciation in the quality ofThomas and Bessemer steels are due to their contents of phosphorus, sulphur, dissolved oxygen, free oxygen, and the high percentage of waste caused by pipes, segregation and blisters. These disadvantages are brought about by the process of manufacture in the converter. The production of sound rolled products also involves a great elimination of waste, thus limiting production and greatly increasing the cost thereof.

Ever since the Bessemer and Thomas processes of refining steels were introduced, these major handicaps in the steels obtained by the blowing process have been present. Therefore a great deal of research has been carried out during the last few years to improve the quality of Thomas steel, and considerable progress has already been made.

Different new and recent processes are at present being applied, but unfortunately they are either very expensive, diflicult to apply, or dangerous. a

The present process of dephosphorization and refining of steel provides a satisfactory solution to these problems in a simple manner.

, metal.

. added chemical salt.

This process produces highly satisfactory results rapidly and at a low cost. Its industrial application is easy and free from danger to either the workmen or the equip ment.

In the various uses of steel, two of the elements which are most objectionable, and which it is most difiicult to eliminate, are phosphorus and sulphur. Their elimination has always been a great worry to steel-makers ever since the establishment of the industry.

The principal methods of refining have as their first object the maximum elimination of phosphorus and sulphur, the presence of which in more or less substantial quantities serves as a basis for the technical and commercial classification of the different qualities of steel.

Aside from the phosphorus and the sulphur, oxygen in ditferent forms, and other included and occluded gases, are also very detrimental. All these elements are found particularly in steels obtained by blowing, and any improvement in their elimination will greatly enhance the value of Thomas and Bessemer steels.

The present invention is for the purpose of dephosphorizing and refining Thomas and Bessemer steels by the use of a mixture of alkali metal salts of great chemical activity. The process at the same time produces a substantial desulphurization.

2,730,442 Patented Jan. 10, 1956 ice The heavy iron and steel industry of Western Europe is based on the rational use of the phosphorus contained in the mineral ores;

Thomas cast iron, which serves to produce Thomas steel, contains on an average 1.80 to 2.00% of phosphorus. This substantial quantity of phosphorus is utilized for its calorific effect in the converter, in the reaction 2P+5/2O2 P205+365.2 cal/moi.

Nevertheless the steel is not completely freed from the initial phosphorus, and still shows traces thereof varying on an average from 0.040 to 0.080%, which is still too high, but one-hesitates to lower them by extreme overblowing, which, if applied in slight excess produces a high oxygen content in the steel, with a detrimental influence on the quality thereof.

The present process is based on the use of an alkali metal salt, containing water of crystallization, namely, crystallized carbonate of soda (NazCOs.10 H2O).

It can easily and without danger be added to the molten In view of the high temperature of this metallic bath, the superheated steam arising from the melting metal rapidly oxidizes the remaining phosphorus. The phosphate thus formed enters intocombination with alkaline oxides produced by'the powerful decomposition of the The elimination of the phosphorus insodic dross is probably effected by the following reaction:

At the present time it is universally admitted in the iron and steel industry:

1) That steam can easily and advantageously take the place of an oxidizing agent in the steel bath, especially as far as phosphorus is concerned;

(2) That the alkali metal oxides NasO and K20 combine with P20 with much greater energy than with lime;

(3) That the alkali metal phosphates have great chem ical stability at very high temperatures.

As regards the free oxides contained in Thomas and Bessemer steels, they are, as a general rule, composed of complex silicates which in a very fluid and reactive sodic slag are eliminated to a great extent and readily decanted.

Tests have been made with a mixture of crystalline hydrated sodium carbonate, NazCoalO H20 and anhydrous sodium carbonate. The latter was added for the purpose of restraining and slowing up the very rapid decomposition of NaaCoal-O H20, a chemical salt with a relatively low melting point. 1

The presence of anhydrous NazCOa, of a rather high melting point, slightly slows up the action of the crystallized soda in such a way that the chemical reaction in the body of the metallic bath can take place under the most favourable conditions.

, The average results obtained were as follows: Dephosphorization 56% minimum Refining (oxides included) 60% Desulphuiization 27% The present invention permits the production of Thomas and Bessemer steels with no more than 0.015 to 0.025% of phosphorus content.

Microscopic examination has shown that steel obtained by this process was of great purity. Any remaining elements were small and fine, and therefore in no way impaired the good quality of the steel.

Chemical analysis and micro-analysis showed the presence of only iron oxides and manganese, oxides, and all complex silicates had disappeared.

Another advantage of this new process consists in the elimination of the gas in the metallic bath. in point of fact, the intense agitation produced in the body of the metal in fusion by the chemical reaction of the mixture eliminates the gases dissolved in the metal.

A partial but rather substantial desulphurization was noticed in the steel obtained by this new process, the afiinity between alkali metal salts and sulphur being very great.

The temperature of the metal was very little affected at the trials .made, owing to the relatively small quantity of the mixture. The process does not require the use of pure products, but only those in industrial use.

The process is carried out in the following manner:

The reactive mixture is added at the rate of 2 (two) kilograms per ton in the Thomas converter at the end of the blowing process and after cleaning.

The retort was raised for a few'seconds to permi the stirring up of the molten metal with the reactive material.

Perm-manganese was then added.

During the flow of the metal into the ladle there was added 3 (three) kilograms of the mixture per ton of steel in order to produce a supplementary effect.

The process is slightly changed for Bessemer steel. No addition of an alkali metal compound can be made in the Bessemer retort, for chemical reasons which are easily understood.

The process for Bessemer steel is as follows: The charge will be blown normally. After careful cleaning, add the ferro alloys. Then transfer the molten metal from the Bessemer converter to the ladle. During the transfer to the ladle, 'whichshould be rather slow, the reactive chemical is added at the rate of 4 (four) kilograms per ton. However, no more than 3 (three) kilograms should be added at one time to the casting during the flow.

These examples are given for illustration only but the invention is in no way limited thereto.

This treatment can be applied to all the processes in the manufacture of steel and in general to the pnrification of any metallic bath.

This new process is economical from every point of view, and the results are certain. The cost of applying this process is about one hundred (100) French francs (20 U. S. cents) per ton of steel.

The main feature of this process consists in the employment of Na2Cos.l0 H2O. The steam that develops at the time of the chemical decomposition of this salt in the liquid metal acts as an ideal oxidizing agent, and it acts principally on the phosphorus.

The residue of the reaction, sodium monoxide, forms a very stable slag, with a very high chemical activity, easily absorbing'the impurities 'containedin the steel.

Crystallized carbonate of soda has not hitherto been used in the steel industry oil-"account of its high percentage of water of crystallization (629 grams per kilogram of NaaCoslO H2O) which might seem dangerous to use. Tests have proved that no danger exists.

The manipulation of the salt is easy and its price is very low.

Aside from the percentages of phosphorus and sul phur, one of the most useful criteria of the quality of steel is constituted by the presence of dissolved FeO, as well as of Mn, S, FeS, SiO2, A1203, in the form of silicates or other complex oxides.

The free oxides are very detrimental, which is explained by the progressive compression of these elements on the edge of the metal grains during crystallization; the discontinuity thus created between the grains considerably reduces the inter-crystalline resistance of the steel.

The new process againbri'ng's about a direct and simple solution in this respect. It'allo'ws the user to produce currently steel containing only a minimum of nonmetallic impurities.

It is very probable that the greater part of the elements (inclusions) mixed with NazO (resulting from the reaction) through the emulsion produced in the ladle, are

an instantaneous removal of these undesired elements.

Numerous macro-graphic tests on steels produced by this new process have shown them to be extremely clean and sound.

The super-refining obtained by this new process permits the production of Thomas and Bessemer steels free from any inherent faults in the manufacture.

This process not only allows of the manufacture of purer and sounder metal, which will be easy to roll, but it enables the productive capacity of a plant using Duplex Thomas and Bessemer electric furnace or Thomas and Bessemer Martin furnace to be increased, and even doubled, while at the same time reducing the work of the personnel.

Thus a substantial decrease in the cost of production is arrived at, and a very important saving in fuel and elcc trical energy.

Compared with other processes having the same purpose in view, the new process presents the advantages of producing a better steel with less expenditure of energy.

I claim:

1. A method of dephosphorizing, desulphurizing and purifying steel, comprising adding to a molten body thereof a mixture of crystalline sodium carbonate decahydratc and anhydrous sodium carbonate, said mixture containing the sodium carbonate in substantial quantity sutiicient to reduce the water evolving rate of the crystalline form of the salt in the mixture, whereby to effect formation ot steam, carbon dioxide and sodium monoxide with consequent reduction of the phosphorus, sulfur and impurities therein.

2. A method of dephosphorizing, purifying and desulphurizing steels as claimed in claim 1, the quantities of crystallized sodium carbonate and anhydrous sodium carbonate in the added mixture being equal to one another.

3..A method of dephosphorizing, purifying and dcsulphurizing steels as claimed in claim I, the addition oi the mixture of sodium carbonates being effected while the molten metal is in the converter.

4. A method of dephosphorizing, purifying and de sulphurizing steels as claimed in claim 1, the addition of the mixture of sodium carbonates being effected partly in the gate of the converter.

5. A method of dephosphorizing, purifying and desuiphuri-zing steels as claimed in claim 1, the addition of the mixture of sodium carbonates being effected partly in the ladle.

6. A method of 'dephosphorizing, purifying and desulphurizing steels as claimed in claim 1, the quantity of mixed sodium carbonates-added being between 2.5 kilograms and 4 kilograms per ton of metal.

7. A method of dephosphorizing, purifying and desulphurizing steels as claimed in claim '1, the quantity ct mixed sodium carbonates added being 3 kiiograrns pcr tonof metal.

8. A method of dephosphorizing, purifying and desulphurizing steels as claimed in claim l, the addition of the mixture of sodium carbonates being effected partly in the converter and partly in the gate thereof, the amount added in the gate being between 10 and 20 kilograms.

9. A method of dephosphorizing, urifying and dcsulphu'rizing steels, comprising the step of adding to thc moltenmetal a mixture of crystallize-cl sodium carbonate containing chemically bonded Water of hydration and anhydrous sodium carbonate in approximate ratio of l to 1 in the mixture in quantity of from 2 to 4 kgs. per ton of steel, and thereby effecting therein the formation of steam, carbon dioxide and sodium monoxide, which with energetic agitation reduces the phosphorus, the impurities and the sulphur contained in the molten steel.

10. A method of dephosphoriz'ing, purifying and dcsulphurizing steels, comprising the step of adding crystallised sodium carbonate (NazCOsJO H2O) to the molten metal, and thereby effecting therein the formation of steam, carbon dioxide and sodium monoxide, Which with energetic agitation reduce the phosphorus, the impurities and the sulphur.

11. A method of reducing the phosphorus content of molten steel in the refining thereof, comprising adding to said molten steel a mixture of sodium carbonates in proportion of approximately 3 kg. per ton of steel, said mixed sodium carbonates containing a substantial quality 10 of crystalline sodium carbonate decahydrate.

References Cited in the file of this patent UNITED STATES PATENTS 1,549,312 Jackson et a1. Aug. 11, 1925 6 Hennig June 19, 1934 Muskat May 17, 1938 FOREIGN PATENTS Great Britain Jan. 27, 1935 Great Britain Aug. 1, 1939 France Feb. 7, 1949 France May 2, 1949 OTHER REFERENCES Evans: Desulphurizing at the Blast Furnace, puband 47. 

1. A METHOD OF DEPHOSPHORIZING, DESULPHURIZING AND PURIFYING STEEL, COMPRISING ADDING TO A MOLTEN BODY THEREOF A MIXTURE OF CRYSTALLINE SODIUM CARBONATE DECAHYDRATE AND ANHYDROUS SODIUM CARBONATE, SAID MIXTURE CONTAINING THE SODIUM CARBONATE IN SUBSTAINTIAL QUANTITY SUFFICIENT TO REDUCE THE WATER EVOLVING RATE OF THE CRYSTALLINE FORM OF THE SALT IN THE MIXTURE, WHEREBY TO EFFECT FORMATION OF STEAM, CARBON DIOXIDE AND SODIUM MONOXIDE WITH CONSEQUENT REDUCTION OF THE PHOSPHORUS, SULFUR AND IMPURITIES THEREIN. 